Process for alleviation of stresses in hardened alloy products



Oct. 27, 1970 J. RAUCH 3,536,540

PROCESS FOR ALLEVIATION OF STRESSES IN HARDENED ALLOY PRODUCTS Filed Nov. 20, 1967 05 3 T5 15 76sec FIG. I 30 I/O-SEPH 1434 um! United States Patent PROCESS FOR ALLEVIATION OF STRESSES IN HARDENED ALLOY PRODUCTS Joseph Ranch, Ivry, France, assignor to LAlumininm Francais, Paris, France Filed Nov. 20, 1967, Ser. No. 684,196

Claims priority, application France, Nov. 21, 1966, 8 0

Int. (:1. 621a 1/30 US. Cl. 148-13 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for relieving residual stresses in hardened metal alloys.

It is well known that metal products may be subjected to a hardening heat treatment for the purpose of improving physical properties and particularly hardness. Heat treatment to harden consists in heating the metal to a temperature above its transformation temperature followed .bY quenching. A product is produced for use at room temperature which is stable at high temperatures. The structural transformation developed during hardening heat treatment result in maximum compressive stresses in the outer portions of the metal part and deeply penetrating tensile stresses whereby the aggregate of the stresses will add up to zero through the cross-section of the .part. V

The phenomenon becomes more pronounced the greater the coefficient the expansion of the metal and the smaller the modulus of elasticity. Thus the stresses are especially high in aluminum alloys, magnesium alloys and copper-beryllium alloys.

In some instances, the stresses produced by hardening heat treatment are advantageous. However, in most instances, they are disadvantageous, especially when the hardened pieces are to be subjected to a machining operation. As a matter of fact, modifications of the residual stresses is effected by removal of outer portions of the metal part which are under maximum compressive stress with the result that change occurs in the final dimension of the part. Removal of such outer layers results in modification of the equilibrium whereby the stresses become unsymmetrical and often cause distortion or even breakage of the metal part. Finally, the residual stresses add to the stresses imposed upon the metal part in use to the extent that safety factors are often lowered.

Attempts have been made to alleviate residual stresses as introduced by hardening heat treatment. For this purpose, a process called inverse hardening has been suggested. In this process, immediately after heat treatment to harden the metal, the part is rapidly reheated so that the temperature at the outer portions will be higher than the temperature at the center. This introduces new stresses of an oppositesign to the stresses in the hardened metal part thereby to superimpose one stress upon the other with resulting attenuation of stress levels.

To the present, such rapid reheating has been carried out by the use of steam characterized by high heat capacity and fluidity. However, even when superheated and compressed, it is not possible with steam to exceed a temperature rise of 100 C. on the surface of the metal part 3,535,540 Patented Oct. 27, 1970 to be heat treated. The heat transfer coefficient is not sufficient to achieve the desired temperature gradient between exterior and interior for completely annulling the residual stresses. As a result, only a somewhat partial diminution is obtained.

Applicant tested bars of aluminum alloy quenched in liquid nitrogen and reheated by projecting steam under pressure onto the surface of the bars. The stress reduction did not surpass even with alloys that are most sensi tive to this type of heat treatment and almost no reduction was achieved with alloys such as aluminum-magnesium-zinc-copper alloys.

It is an object of this invention to provide a method for heat treatment of hardened metal parts of the type described in which a substantial alleviation of stress levels can be achieved and it is a related object to provide an inverse hardening process in which stress levels of hardened alloy metals can be markedly reduced and even eliminated.

The invention is characterized by heating the external portion of the hardened metal part extremely rapidly by induction heating in an induction furnace. The speed at which the metal is heated, and therefore the temperature gradient between the exterior and the interior of the metal part, can be made very large by utilization of a furnace having suitable power. The final temperature to which the metal is reheated can be regulated at will and the reheating process can be conducted in a continuous process without difficulty. In most instances, it will be unnecessary to cool the metal to below ambient temperature before heating by induction for inverse hardening in accordance with the practice of this invention to provide a suitable temperature gradient. This is because the temperature rise is generated within the part itself as distinguished from heat transfer. By induction heating of the outer portions of the metal part, it is possible to achieve a temperature gradient within the range of l- 200 C. with a rise in temperature in the external or tions of only 250-300 C. in a matter of a few seconds. For this purpose, the superficial specific power transmitted by the inductor is preferably within the range of 0.5 to 2 kw. per square centimeter of surface to be treated.

However, with alloys of the type having high stresses and which are difiicult to eliminate, it is conceivable that heating by induction be preceded by cooling the metal to a temperature below ambient, such as between ambient temperature and the temperature of liquid nitrogen at atmospheric pressure. Under such circumstances, the temperature gradient may be maintained for a longer period of time than when starting with ambient temperature to achieve the desired alleviation of stresses in such metals in which stresses are had to eliminate.

FIGS. 1 and 2, accompanying this application, are diagrams showing external temperature (TE) and internal temperature (TI) as a function of time and difference (TET I It is possible by the treatment of this invention completely to cancel the stresses. It is even possible to reverse the direction of the stresses to substitute tensile stresses for compressive stresses in the outer portion of the metal part, etc. Such inversion of stresses is promoted by tempering after induction heating has been completed. This reversal is often undesirable and can be avoided by quenching the metal part, as by sparging with water 10 to 40 seconds after completion of induction heating.

The choice of inductor frequency will depend somewhat upon the dimensions of the metal pieces to be treated. With large pieces requiring higher power, the current is produced by rotating machines having a fre quency which is generally within the range of 1 to 10 kilohertz (kHz.). With thin pieces, it is desirable to limit the penetration of the current, that is, the distance within the external region of the part wherein the thermal release is generated. For this purpose, use is made of machinery having high frequency and a capacity of about 100 kHz. for small penetration. Such current is generated by electronic means. Thus it is possible to determine the conditions of induction current strength and frequency for producing a temperature gradient which will completely cancel or markedly reduce the residual stress levels.

In accordance with a preferred mode of carrying out the invention, the part being treated is rotated such as at 100 to 400 rpm. during induction heating. This avoids irregularities caused by poor centering in the inductor or a symmetry fault while in the inductor. When heat treatment by induction heating does not result in sufficient decrease of stresses, it is possible to repeat the treatment to achieve a total or almost total cancellation of the stresses.

With alloys that necessitate thermal treatment for natural aging or tempering, such treatment is normally applied after the described stress relieving process.

The following examples are given by way of illustration, but not by way of limitation, of the practice of this invention.

The test pieces used are cylindrical bars of various aluminum alloys dimensioned to have a length of 100 mm. and some of which have a diameter of 30 mm. while other have a diameter of 60 mm. For the determination of stresses, the freshly hardened and machined bar is placed in a horizontal position and a comparator is used to measure the bend of a certain number of generatrices in order to determine the one G1, which has a minimum bend. Then, taking as the apices the generatrix G2, diametrically opposed to G1, cross-cuts are made with regularly spaced saw marks along the whole length of the bar with each of the saw cuts extending to the axis of the bar and the generatrix G1 is measured again. From the difference between the algebraic values of the bend in millimeters, before and after, it is possible to determine the internal stress of the bar. Conventionally, this difference is designated by the word stress. In order to evaluate the eflect of the treatment to relieve stresses in accordance with the practice of this invention, the stress of a control bar which has undergone hardening treatment only, is compared with the stress of identical bars which after hardening heat treatment have been subjected to treatment for the elimination of stress in accordance with the practice of this invention. To express the result, use is made of the following equation called residual stress:

stress after relaxation (The arbitrary abbreviation Rs has been selected to replace the French abbreviation Cr for contrainte residuelle which otherwise might be mistaken for the chemical symbol of chromium.)

The inductor used for heating has a power of 120 kw., corresponding to a superficial specific power of 0.5 kw./ cm. for the test pieces of 60 mm. diameter and 1 kw./cm. for the 30 mm. diameter test pieces. The frequency of the inductor is 8 kHz.

The heat impulses were for the following duration:

Heating starting with room temperature 1.4 sec. for d.=30 mm. Heating starting with room temperature 3 sec. for d.=60 mm. Heating starting with the temperature of liquid nitrogen 2.7 sec. for d.'=3'0 mm. Heating starting with the temperature of liquid nitrogen 7 sec. for d.=60 mm.

The temperatures reached in the exterior zones were about 275 C. and the maximum temperature difference 4 between the exterior portion and the interior portion was of the order of 180 C. The examples included the following aluminum alloys:

Afnor AZ8GU: 8% Zn, 2.7% Mg, 0.6% Cu in 30 mm.

bars,

Afnor A-SGM: 1% Si, 1% Mg, 0.5% Mn in 30 mm.

bars and 60 mm. bars,

Afnor AU4G1: 4% Cu, 1.5% Mg, 0.5%Mn in 60 mm.

bars,

Afnor AS10G: 10% Si, 0.5% Mg in 30 mm. bars,

Afnor A-USGT: 4.5% Cu, 0.2% Mg, 0.2% Ti in 30 mm.

bars.

EXAMPLE 1 30 mm. bars of alloy A-SGM which have been given hardening heat treatment only show a stress of 0.10 when heated from ambient temperature with a kw. inductor for 1.4 seconds and cooled in open air, an inversion of stress which goes from 1.1 to -0.02 is obtained corresponding to a value of Rs+20% In a second test in which the bar is cooled with water 25 seconds after the end of the induction heating, a total cancellation of stress is obtained.

EXAMPLE 2 60 mm. bars of the same alloy, after hardening heat treatment only, give a stress of 0.075. After induction heating for 3 seconds and cooling with water for 15 seconds a residual stress is obtained corresponding to the value Rs=15%. The treatment is repeated a second time whereupon the stress is completely cancelled.

EXAMPLE 3 60 mm. bars of alloy AU4G1, after hardening heat treatment only, show considerable stress of 0.14. After heating from room temperature and cooling in air in the manner previously described, the stress is reduced to 25% of its initial value. A repetition of the treatment lowers the stress to 5% of its initial value.

EXAMPLES 4 AND 5 30 mm. bars of the alloy A-USGT have a stress of 0.075 after hardening heat treatment only. The stress drops to 10% of this value by heating as previously described for 1.4 seconds and cooling in air. Such lowering to 10% of the initial stress is ordinarily suflicient under practical conditions.

In another test the bar is cooled to the temperature of liquid nitrogen and then reheated in the inductor for 2.7 seconds followed by cooling with water 15 seconds later. The stress was reduced to 20% of its initial value and was completely cancelled when the heat treatment was repeated a second time.

EXAMPLE 6 30 mm. bars of alloy A-SlOG which have been subjected to hardening heat treatment only show a rather small stress of 0.04. Heat treatment for 1.4 seconds from ambient temperature, followed by cooling in air, inverts the stress to a value Rs=-75%. In another test, the bar was cooled 15 seconds after induction heating and the stress was found to be completely cancelled.

EXAMPLE 7 30 mm. bars of alloy A-Z8GU had considerable stress of 0.21 after hardening heat treatment and the stress is difficult to eliminate. Heating for 1.4 seconds from ambient temperature followed by air cooling gives a result whereby the stress is reduced to 43% of the initial stress.

In another test the metal is cooled to -196 C. and then reheated for 2.7 seconds and water cooled 10 seconds after induction heating has been completed. The stress is lowered to 18% of the initial value from the hardening heat treatment only. The stress is cancelled completely by repeating the induction heat treatment two times.

Instead of waiting 10 seconds, one may wait 30 seconds before cooling, and if one repeats this treatment two times, an inversion of stress is obtained with a value of Rs=l%. The latter result is not desirable per se. However, it shows the effectiveness of the process of this invention since it shows that it is capable of inverting the direction of the stresses in an alloy which, after hardening heat treatment, has stresses which are difficult to eliminate. For purposes of comparison, an attempt was made to accomplish inverse heating by the use of steam under 2.5 kg./cm. pressure for heating 30 mm. bars of the alloy AZ8GU after hardening heat treatment and cooling to 196 C. The heating time is of the order of 60 seconds or ten times longer than the heating time by induction. After treatment with steam followed by cooling with Water, the stress was practically unmodified corresponding to a value of Rs=98%.

It will be apparent from the foregoing that I have provided a most eflicient and novel means for stress reduction of metal alloys. after hardening heat treatment whereby substantial reductions can be achieved in a simple and efficient manner to the extent that stress levels can be substantially eliminated or even reversed in their sign.

It will be understood that changes may be made in the details of conditions for operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In the process for the alleviation of stresses in solid parts of hardened metal alloys selected from the group consisting of an aluminum alloy, a magnesium alloy and an alloy of copper and beryllium comprising the steps of exposing the metal part to induction heating of high frequency for a short period of time whereby heat generation is concentrated in the external portions of the metal part to provide a substantial temperature differential between the exterior and interior of the metal part, and then cooling the metal part.

2. The process as claimed in claim 1 in which induction heating is effected with a high frequency inductor having a specific power within the range of 0.5 to 2 kw.

6 per square centimeter of surface of the metal parts to be treated.

3. The process as claimed in claim 1 in which the cooling step is effected by cooling in air or water commencing 10 to 40 seconds after induction heating has been completed.

4. The process as claimed in claim 2 in which the frequency of the inductor is within the range of 1 to kHz. with the frequency being proportional to the thickness of the metal part.

5. The process as claimed in claim 1 which includes the step of rotating the metal part about its axis during induction heating.

6. The process as claimed in claim 5 in which the metal part is rotated at a rate within the range of 100 to 400 rpm.

7. The process as claimed in claim 1 in which the induction heating to relieve stresses follows substantially immediately after hardening heat treatment of the metal part but after the temperature has been lowered to about ambient temperature.

8. The process as claimed in claim 1 in which the induction heating to relieve the stresses follows substantially immediately after hardening heat treatment of the metal part but after the step of cooling the metal part from hardening heat treatment to a temperature within the range of ambient temperature to the temperature of liquid nitrogen.

9. The process as claimed in claim 1 in which the metal part is formed of an aluminum alloy and in which induction heating is carried out in a manner to raise the temperature of the exterior portion of the metal part to within the range of ZOO-250 C. within 1 to 10 seconds.

References Cited UNITED STATES PATENTS 2,604,419 7/1952 Herbenar 148-13 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 

